Among steel elements for rubber reinforcement there are mainly used brass-coated wires and galvanized wires, including cables, cords, fabrics and other products made from the wires. Brass-coated steel wires and steel cords have found the widest variety of applications as strengthening elements in vulcanized rubber products and especially in radial tires, because of their attractive vulcanization bonding properties and their elevated mechanical strength, which often may exceed 2000 N/mm.sup.2. Typical examples include bead wire (up to 2 mm diameter), medium to fine cable and hose wire (up to 0.8-1.0 mm diameter) and especially tire cord made from fine steel wires (0.1 to 0.4 mm diameter) having a tensile strength of at least 2400 N/mm.sup.2. Such brass-coated wires are normally produced from plain carbon steel with a carbon content of at least 0.4% by a wire making process which usually involves (after prior deformation) a patenting step to form a pearlitic wire structure, a wire drawing operation to obtain a required final wire diameter and tensile strength and a coating operation to provide the wire surface with a rubber adherent brass layer of appropriate composition and thickness (usually a CuZn-alloy with not less than 60% copper and a coating thickness below 0.50 micrometer).
Prior art pearlitic steel wires with or without a rubber adherable surface coating are not immune to hydrogen embrittlement and may fail by sudden brittle cracking or by corrosion fatigue degradation according to the prevailing stress and environmental conditions in service. This is particularly true of brass-coated steel wires or steel cords exposed during tire usage to humidity or other corrosive agents, which cannot be avoided in harsh driving conditions. The situation becomes even worse when steel wires of higher than normal strength are employed.
In this respect it is now generally acknowledged that common rubber adherent coatings, and in particular thin brass coatings, cannot afford adequate protection of the steel wire substrate against hydrogen penetration and resulting embrittlement. Indeed, incidental coating imperfections or damaged areas due to handling are not always easy to avoid. In addition, fretting damage occuring as a result of cord usage will inevitably cause local penetration of the coating.
Recently, a strong desire to raise useful wire and cord strength has created an even stronger need for preventing premature wire and cord breakage in some applications, especially in heavy-duty tires. Hence, there is a clear demand in industry to increase the resistance to hydrogen embrittlement and corrosion fatigue of current brassed wires and cords of conventional strength to ensure a more durable reinforcement and to achieve a longer tire life. Moreover, there is a great need for imparting better protection to high-tensile strength wires and cords, with tensile strength levels in excess of 3000 N/mm.sup.2 against rapid and unpredictable cracking by hydrogen embrittlement and against early deterioration by corrosion fatigue and stress corrosion effects, given the most unreliable cracking behaviour of current high-tensile strength reinforcement wires in severe loading conditions and particularly under conditions of humidity and corrosive attack.
In order to overcome some of the above-mentioned shortcomings of current wires or at least to reduce the severity thereof, various proposals have been made in the recent past. Among these proposals are measures intended to improve the protective capacity of the rubber adherent brass coating, e.g. by using a compact layer of increased thickness and attempts to develop a more corrosion resistant brass alloy composition, e.g. a ternary alloy such as CuZn-Co, CuZn-Ni, etc. It has also been attempted to seal the brass surface or to modify it by chemical means, by metallic top films and/or by applying corrosion inhibiting compounds. In spite of the fact that these measures may decrease the surface corrosion rate to some extent, it has been observed that hydrogen embrittlement is not yet sufficiently suppressed. Furthermore, there can occur undesirable side effects on rubber adhesion.
Other proposals often relate to the selection of corrosion resistant alloyed steel compositions as a wire material; however such materials are more expensive and frequently cause difficulties in heat processing and drawability of fine wires.